Art of mounting piezoelectric crystals



March 9, 1943. w. R. DOHAN 2,313,129t

ART OF MOUNTING PIEZOELECTRIC CRYSTALS Filed Jan. 51, 1941 Patented Mu. s, 1943 ABT F MOUNTING PIEZOELECTRIC OEYBTAIB `William B. Dohan. Wynnewood, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application January 3l, 1941, Serial No. 376,874

(Cl. lll-327) 5 Claims.

My present invention relates to the art of mounting piezoelectric crystal elements, particularly those constituted of Rochelle salt (sodiumpotassium tartrate) .and has for its principal object to provide improvements in moisture-'proof mounts for such piezoelectric crystal elements.

Other objects and advantages will be apparent and my invention itself will be best understood dehydrate; at humidities higher than 87.2 percent they will slowly dissolve. In either event the useful piezoelectric properties of such crystal elements are destroyed.

Since the above described conditions of humidity may be encountered (particularly in tropical countries) in normal use, various means have heretofore been proposed for protecting Rochelle salt and similar crystal elements from excessive hydration and dehydration. By way of example, it has previously been proposed to enclose the crystals in hermetically sealed envelopes and/or to coat them with certain asphaltic or wax compounds. However the necessity of transmitting mechanical vibrations to the crystal or crystals (as in phonograph pickups") renders it dimcult to maintain a hermetic seal about the crystal and, as will hereinafter more fully appear, conventional rigid coatings lprovide but little real protection for the crystal. In this latter connection it may be observed that to increase the thickness of a wax-like or asphaltic coating with a view to increasing its protective qualities may result in undue damping of the crystal vibrations.

I have discovered that the foregoing objections to the moisture-proof coatings of the prior art are substantially' obviated by the use of certain semisolid or semiiluid embedments. Thus. I may surround, immerse or em'bed a Rochelle salt or similar crystal elementor elements, and preferably the electrodes therefor, in petroleum, or in lanolin, or in a mixture comprising petrolatum and lanolin or other water emulsifying agent, such for example as calcium stearato or aluminum stearate. l Preferably these moisture-proofing substances or mixtures are applied in the form of a filling to the casing or cartridge in which the crystal assembly is mounted, in which case the crystal and its electrodes are allv preferably immersed in and surrounded by the illling.

While a Rochelle salt crystal which is mounted and operated in accordance with my invention in petrolatum exhibits a substantially higher factor or merit than one provided with an asphaltic coating. I have achieved very much better results when the embedment material is constituted in whole or in part of lanolin or other emulsifying agent. By way of example: under similar yextreme conditions (90 percent relative humidity) a Rochelle salt crystal element coated with an asphaltic compound exhibited a useful life of 24 hours, one embedded in petrolatum a useful life of 300 hours, while one embedded in lanolin was apparently unaffected at the expiration of 500 hours. The failure of the first mentioned crystal may have been due to minute voids, inherent in the granular structure of asphaltic coatings, through which moisture seeped 4and destroyed the crystal. 'I'he eventual failure of the crystals which were embedded in petrolatum may be attributed to the factvthat moisture settling on the petrolatum eventually worked inwardly through the mass and was absorbed by .the crystal.

I attribute the vastly superior protective action of the lanolin, and of the other water emulsifying agents, to the fact that these materials hold themoisture in the dispersed phase. That is to say, the ilne particles or globules of water apparently become coated with the embedment material so that if or when these coated liquid particles reach the region of the crystal, the coatings (i. e., the continuous phase of the emulsion) on these discrete particles inhibit or prevent actual contact of the water with the crystal so that it can not be absorbed by the crystal.

Of the emulsifying agents, above mentioned, I prefer lanolin (lanum anhydrous U. S. P.), as this material has a melting point of 31 to 34 C. and has a viscosity such that it serves not only to damp-out resonance peaks. (at temperature of the order of 20 to 25 C.) but also serves to compensate for the usual drop in the crystal output at temperatures above 20 C., by reason of its decreased viscosity at increasing temperatures. Further, lanolin is capable of absorbing up to percent of its weight of water although it is not water soluble. The advantages of my invention, however, may be achieved to a greater or less degree, in the above respects, with any of the aforementioned or equivalent semiliquid or semisolid materials, either alone or in combination.

In the accompanying drawing I have shown the invention as applied to a conventional phonograph pick-up comprising a casing I which encloses a piezoelectric unit comprising a pair of Rochelle salt crystals 3 with which are associated terminal foils B of conductive material in a well known manner, the crystal unit 3 being spaced from the casing I as by the clamps 'l in which it is supported, or otherwise. As in standard practice the casing i also carries a torsion yoke 9 which extends through a suitable bearing i I, and. the outer or free-end of the yoke 9 being provided. with a needle socket I3. In accordance with my invention the casing is provided with a illling comprising a semisolid or semiliq'uid mass which is preferably constituted of one of the insulating materials or compoundsl above described and within which the crystal unit 3 is adapted to be vibrated as by the force transmitted thereto through the yoke 9. l

Various modifications of my invention will suggest themselves to thosel skilled in the art. It t to be understood, therefore. that the foregoing should be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit oi the appended claims.

I claim:

l 1. A piezoelectric unit comprising a. casing containing a piezoelectric crystal element and a semisolid mass of insulating material in which said crystal is embedded, and means extending from said crystal throush said insulating material to the exterior oi said casing for applying an operating force to said crystal.

2. The invention as set forth in claim 1 and wherein said insulating material comprises a water emulsiiying agent.

3. The invention as set forth in claim 1 and wherein said insulating material comprises lanolin.

4. The invention as set forth in claim 1 and wherein said insulating material comprises a water-insoluble metallic soap.

5. The invention as set forth in claim 1 and Jvltierein said insulating material comprises petro- WmusMnDoHAN. 

